I've just started using Heroku with Django and it seems great. However, when I change my existing models I'm not sure how to run those changes to the Heroku environment. The syncdb works just fine when adding all new database tables, but how should I modify existing tables?
I found out that Heroku provides psql access only to a dedicated database so that's out of the question. I haven't tried South but it seems like a solution.
So I guess I'm asking how to make database changes with Django and Heroku?
What you are asking for is called "schema migration" or even "schema evolution". Django has some documentation about it on the wiki.
Django's syncdb command does not support that. As a matter of fact, the documentation for syncdb is clear:
Creates the database tables for all apps in INSTALLED_APPS whose
tables have not already been created
Rather, django proposes to use drop the tables manually and then to run syncdb again in the documentation of the deprecated reset command:
You can also use ALTER TABLE or DROP TABLE statements manually.
But fear not, there are many reusable apps to help you with proper schema migrations and hopefully you can pick the one that suits you best. Rather than elaborate in my answer, please let me link an article I wrote about Django schema migration which compares all current solutions.
South works great on Heroku.
Related
I have a flask app that is deployed on Google's App Engine. I have noticed a minor bug and I would like to fix it but my database is already populated.
How can I make this minor code change and push / deploy back to my app without losing all my data? (which is probably a basic question but I'm not finding much. all tutorials online are focused on creating the app and deploy, not updating)
Thus far, I have been dropping and re-creating the tables whenever I redeploy, mostly out of ignorance. Here are the steps I have followed
1). make the change in my app
commit and push changes to bitbucket source code
in Google Cloud SDK: git pull
Google Cloud SDK: gcloud app deploy
These steps result in an empty database because the directory I am pushing from my local computer has an empty database. Is this where I should be using git merge?
Is this a database "migration" or is this a "git merge"? I'm not sure what the right terms are to use to research this further. Thanks.
There are a couple of angles to your question. I'm going to try to give you some information, but let me warn you, this isn't going to be a trivial change to your workflow, you'll have to change some things.
First of all, based on the way you worded your question I get the idea that you commit your database to git along with your code. If I got this right, then this is something that you need to stop doing. The database is not code, so it should not be committed to source control.
You should have a completely independent database on each installation of your application. For example, you will have a database on your own machine to do development. You will also need another database in your gcloud deployment. You may need more databases if you have other uses for your application. A very common third database for many people is one that is used for automated tests, which could also be located in your local development machine, but is not the same database that you use for day to day development.
To make changes to your database schema you will not drop and recreate tables anymore, that is clearly something that you already realized that needs an improvement. A good approach to make these changes is to use a database migration framework. These tools allow you to generate short scripts that make these changes to the database in a more focused way, without destroying and recreating everything, and for that reason, the data is in general not lost. For Flask-SQLAlchemy, the best option for database migrations is Flask-Migrate, which is a lightweight wrapper around the Alembic migration framework. (I might be biased here as I'm the author of the Flask-Migrate extension!).
Documentation for Flask-Migrate: https://flask-migrate.readthedocs.io/en/latest/.
I have been working on developing the models for a Django app. I am working on it in my spare time. I have an issue when it comes to testing. Whenever I realize I have a mistake in my models, I have to go through the trivial but annoying process of dropping the corresponding database and then recreating it and running python manage.py syncdb. Apparently, this is because Django's syncdb cannot change the database schema. I do not fully understand what that means. Although, I know it is a pain to have to keep deleting and recreating my database every time I try and change something in my models.
Is there a better way of doing this? Is something wrong with my install? I feel like this is such a simple thing to fix that I must be doing something wrong.
No, nothing is wrong with your installation. It is a limitation of django.
There is a 3rd party app called django-south, which can be used for managing "migrations" (changes to database models). It is a very widely used app for managing database changes.
There are lots of documentation online to help you understand how south works, and how to use it.
This is a good tutorial on south
I've inherited a v 1.0 Django/Python intranet site.
Django normally pushes from the Django code into the database. For say, table definitions (DDL).
Db architects associated with the project have a new database design that they want me to hook up to the site.
That is, I need somehow to pull the DDL defintions from out of the db and into Django code. E.g. some way to generate a new class from a table.
I know a lot of python but much less Django. Any ideas on how this might be done?
Yes, Django can work with pre-existing DB's. Check this out.
Basically, the command you need to pull models from the DB is python manage.py inspectdb > models.py
I'd like to respect the database as a "read-only" and never write to it. Is there a way to easily prevent syncdb from even bothering to check to update the database?
With Django 1.2 and the ability to have multiple databases, it'd like to be able to query a database for information. I'd never need to actually write to that database.
However, I'd be scared if syncdb ran and attempted to update that database (because I may not have a technically read-only account to that database). Mainly, I'd just like to use/abuse the Django ORM as a way to query that database.
UPDATE: Sorry, I need to be able to sync one of the databases in settings.py, just not this specific one.
Heh, I guess I'll answer my own question (RTFM!)...
http://docs.djangoproject.com/en/dev/topics/db/multi-db/#an-example
def allow_syncdb(self, db, model):
...
That's a definite start...
If you don't need syncdb, don't run it, simple as that. Updating the database is what it does, so if you don't need that, you shouldn't run it - it doesn't do anything else.
However if you're actually asking how to prevent syncdb from running at all, one possibility would be to define a 'dummy' syncdb command inside one of your apps. Follow the custom management command instructions but just put pass inside the command's handle method. Django will always find your version of the command first, making it a no-op.
This issue came up for me when working with read-only mirrors of Microsof SQL Server databases (uhhg). Since you can't selectively run syncdb on a single app or database. But you have to run syncdb when you first create a new Django project or install an new app that requires it (like south). What I did was to put my read-only database in its own Django app and then add an empty South migration to that app. That way syncdb thinks south is handling db setup for those apps, and south doesn't do anything to them!
manage.py schemamigration ap_with_read_only_database --empty initial_empty_migration_that_does_nothing
That leaves you free to manage the schema of that db outside of django.
I'm in the process of starting up a web site project. My plan is to roll out the site in a somewhat rudimentary form first and then add to the site functionality along the way.
I'm using Subsonic 3 for my DAL, and I'm expecting the database will go through multiple versions as the sites evolve. This means I'll need some kind of versioning and migration tools. I understand that Subsonic has built in migration possibilities, but I'm having difficulties grasping how to use these tools, in my scenario.
First there's the SimpleRepository model, where the Subsonic "automagically" handles the migrations as i develop my site. I can see how this works on my dev-machine, but I'm not sure how to handle deployments with this.
Would Subsonic run the necessary migrations on my live site as the appropriate methods are called?
Is there some way I can force all necessary migrations on a site while taking the site offline, when using the Simplerepository model? (Else I would expect random users to experience severe performance cuts, as the migration routines kick in)
Would I be better off using the ActiveRecord model, and then handling migrations with the Subsonic.Schema.Migrator? (I suspect so)
Do you know of any good resources explaining how to handle this situation with the migrator? (I read the doc, but I can't piece together how I would use this in practice)
Thanks for listening/replying.
Regards
Jesper Hauge
I would advise against ever running migrations against a live site. SubSonic's migrations are really there to make development simpler and should never be used against a live environment. To be honest even using SubSonic.Schema.Migrator you're still going to bump into the fact that refactoring databases is an incredibly hard problem. For example renaming a column in a table using management studio is trivial, but what happens in the background involves creating an entirely new table and migrating all the constraints, data etc. before renaming the new table.
The most effective way I've found for dealing with this is:
Script all database changes as you make them in your development environment (SQL Server Management Studio will do this for you) and add these scripts to your source control.
As part of deployment (obviously backup first) run the migration scripts and then deploy the updated application on success.
Whether you use ActiveRecord or SimpleRepository is then down to whether you want the extra features/complexity of ActiveRecord.
Hope this helps
i would use activerecord easy to use and any changes you just run the TT files, you would then just build or publish your slution and done ???? SVN will keep your multiple versions of the build stage so if you make a tit of it you just drop back a revision.